1. Field of the Invention
The present invention relates to a clamp level adjusting circuit for making a black level correction to an output from an image pickup device in an electronic camera or the like. The invention also relates to an image processing apparatus and an image processing program for processing photographic images captured by an electronic camera.
2. Description of the Related Art
It is conventionally known that outputs from an image pickup device of an electronic camera contain black level noise which results from dark current components in the light-receiving elements. There is a method for correcting such black levels which is well known to those skilled in the art. The method includes the steps of detecting the black level of a lightproof area or part of the image pickup device shielded against ambient light and then subtracting the black level from the image level of an unshielded effective pixel area.
Disclosed in Japanese Unexamined Patent Application Publication No. 2003-209713 is a black level correction apparatus for comparing the average value of horizontal rows of digital signals in the lightproof area with a black level target value to calculate a correction value and then providing feedback of the digital-to-analog converted correction value for a black level correction.
Suppose that there is a plurality of defective pixels in a particular horizontal row in a lightproof area. In this case, the apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2003-209713 mentioned above could be improved in that a significant variation in black level average value from a proper value would cause the black level correction to be locally reduced in accuracy.
Furthermore, according to Japanese Unexamined Patent Application Publication No. 2003-209713 mentioned above, no information on the black level correction exists in the image data that has been corrected. It is thus very difficult to determine based on the image data as to how the black level correction was made. Particularly when the corrected image data is processed to adjust the brightness of the image, the information on the black level correction would serve as a reference on which the brightness of the image could rely when adjusted and thus provide a clear range of allowable correction. However, according to Japanese Unexamined Patent Application Publication No. 2003-209713 mentioned above, no information on the black level correction was not included in the image data. Accordingly, the user had to make black level re-corrections repeatedly by trial and error. With this being the case, conventional computer image processing did not sufficiently meet user requirements for subtle brightness adjustment of image data.
On the other hand, recent years have seen increasingly widespread use of electronic cameras which include an image pickup device for photoelectric conversion of subject images to create photographic image digital data (photographic image data). It has also been commonly practiced to adjust various parameters of photographic image data on a computer to create images as desired by users.
In this context, an image playback apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2000-66312 can be used for playback of digital images on a computer. To this end, the apparatus is adapted to associate supplementary information on shooting circumstances, such as dates and places of shooting, with the photographic image data for transmission to the computer.
In general, outputs from an image pickup device of an electronic camera contain black level noise caused by dark current components. The electronic camera is thus adapted to detect the black level with part of the image pickup device being shielded against ambient light in order to make a black level correction by subtracting the detected black level from the image level. Accordingly, when photographic image data is processed on a computer to adjust the brightness of the image, the information on the black level correction made by the electronic camera would serve as a reference on which the brightness of the image could rely when adjusted and thus provide a clear range of allowable correction.
However, conventionally, no information on the black level correction was left in photographic image data that was created by the electronic camera. Accordingly, when the user desired to process images on a computer, it was very difficult to determine based on the image data as to how the black level correction was made. According to Japanese Unexamined Patent Application Publication No. 2000-66312 described above, no information indicative of the contents of image processing performed by the electronic camera was contained in the supplementary information. Accordingly, the user had to adjust the brightness of images on a computer repeatedly by trial and error. With this being the case, conventional techniques did not sufficiently meet user requirements for subtle brightness adjustment of image data on a computer.